add isArray.js marker for interop with other js libraries

Add a marker to make array compatible with other libraries
via either the isArray-polyfill or my libraries willing to
support isArray.js container objects themselves.

An example of this allowing interop with leaflet.js is here:

http://bl.ocks.org/dribnet/4326899

More information available here: 

https://github.com/dribnet/isArray.js